Corrosion resistant low-alloy steel



Aug- 5, 1959 AKIRA TERAMAE ETAL 3,

CORROSION RESISTANT LOWHKLLOY STEEL Filed March 23, 1966 2 Sheets-Sheet 1 O O O O O f m N Corrosion Rate By Weight Loss (mg/cm) CORROSION RESISTANT LOW-ALLOY STEEL Filed March 25, 1966 2 Sheets-Sheet 2 e123 uogsouog TempfC) 0 Conc.( O 10 2O 40 50 70 80 United States Patent CORROSION RESISTANT LOW-ALLOY STEEL Akira Teramae, Kyozo Shinmyo, and Satoshi Kado, Tokyo, Yasuo Otoguro, Sagamihara-shi, lkuya Noda, Kamaishi-shi, and Tsugio Ikeda and Shoichi Nakanishi, Himeji-shi, Japan, assignors to Fuji Iron & Steel Co., Ltd., Tokyo, Japan Filed Mar. 23, 1966, Ser. No. 536,914 Claims priority, application Japan, Mar. 25, 1965, 40/ 17,312 Int. Cl. C22c 39/54, 39/30 U.S. Cl. 75-125 4 Claims ABSTRACT OF THE DISCLOSURE Low alloy steel which is resistant to non-oxidizing acids. It consists essentially of not more than 0.80% of carbon, 0.201.50% of manganese, not more than 1.00% of silicon, 0.01-0.15 of antimony, not more than 0.03% of sulphur, not more than 0.030% of phosphorus, 0.08- 0.60% of copper, the balance being substantially iron. The copper and antimony produce a synergistic eflfect on the resistance to the alloy to such acids.

This invention relates to low alloy steels to which has been added a small amount of antimony other than the ordinary additives to improve their corrosion resistance, and such low alloy steels have an excellent corrosion resistance, particularly against non-oxidizing acid.

Although precious metals, such as gold and platinum, have excellent acid resistance, they are extremely costly and their commercial utility is very limited.

Other than the precious metals, metals such as niobium, tantalum, zirconium and titanium, and alloys such as nickel base alloys are also excellent in acid resistance, but these metals and alloys are also considerably costly and their commercial application is thus limited.

Of stainless steels, some grades of austenitic stainless steels to which has been added relatively large amount of silicon, silicon together with copper, molybdenum together with copper, molybdenum together with silicon or molybdenum together with antimony are widely known to have resistance against non-oxidizing acids.

Of cast irons, grades containing relatively large amount of silicon or nickel are known as acid resistant cast iron.

Recent developments of industries are requiring more and more metals and alloys which can be used in corrosive environments containing non-oxidizing acid. Any of the above mentioned metals and alloys are considerably costly or have difiiculties in working and their commercial application has been limited extremely by economical factors.

For this reason, alloy steels containing small amount of nickel, chromium and copper have been predominantly used in large tonnage in certain applications of similar environment where requirement for acid resistance is not so severe.

To illustrate one example of the typical applications in which these alloy steels have been used, brief explanation will be set forth in regard with an air preheater.

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Large part of the fuel consumed in boilers of heat power plants and in other industries is occupied by heavy oil and coal. Heavy oil in general contains more sulphur than coal and the sulphur content varies up to 5% depending on the origin. The content of sulphur dioxide gas in exhaust gas from combustion of heavy oil is said to be proportional to the sulphur content in heavy oil, and this sulphur dioxide gas changes into S0 by contact oxidation or direct chemical combustion and S0 in turn reacts with water to form sulphuric acid. Sulphuric acid formed through such process is highly concentrated and causes so called sulphur dew point corrosion on the air preheater of a boiler in which heavy oil is used.

Therefore a preheater made of conventional Ni-Cr-Cu low alloy steel has been met with large problems of corrosion of equipment, thermal efliciency, safety and operation rate. To overcome these problems various studies have been done with regards to the use of heavy oil. One of proposed methods, in which the amount of air to be used for combustion is reduced, is said to be effective for reducing the sulphur dew point corrosion, but this method can not be entirely reliable. Meanwhile a method for removing the sulphur content in heavy oil has also been under investigation, but successful results have not yet been obtained.

Therefore, up to now, development of new materials which provide better resistance against sulphur dew point corrosion than carbon steels and Ni-Cr-Cu low alloy steels, and which can be available at low price has been eagerly wished.

Similar things may be said in case of exhaust tubings in internal combustion engine systems and flues. Similarly, development of low priced steel materials having strong resistance against corrosion by hydrochloric acid has been wished for use in apparatus and constructions in the petroleum industry and other chemical industries where hydrochloric acid is formed by decomposition of chlorides.

Many investigations have been conducted on corrosion problems of steels in non-oxidizing acid, and recently E. Williams and M. E. Komp observed in Corrosion vol. 21, No. 1, p. 9 (1965) that copper content in steel is one of the important factors controlling the corrosion rate. E. Williams et al. measured the corrosion rate of carbon steels in 42 percent sulphuric acid and discovered that corrosion rate of steel containing 0.02% of copper, for example, showed sixteen times larger than that of steel containing 0.1% of copper and no substantial difference in corrosion rate was observed for steels containing 0.1% to 0.16% of copper.

Therefore it is a known art to add about 0.1 of copper in steel for improving corrosion resistance against nonoxidizing acid. In fact copper content coming into steel from scraps and so on during steelmaking process is about 0.1%. This means that the study of E. Williams et al. did not discover entirely new corrosion resistant materials.

Therefore, up to now the development activities for low cost, low alloy steels which are used in large tonnage and have good corrosion resistance have been made on the basis of these copper-containing steels (copper content is more than 0.1%). For example a method has been proposed for preventing corrosion in non-oxidizing acid such as sulphuric acid by the combined effects of copper and 3 tin or arsenic with the addition of tin or arsenic together with copper (less than 0.6%) as disclosed in a Japanese patent application filed by Sumitomo Metal Industries (Publication Number 8110 39-28011).

In contrast to the above arts, the present invention is featured in that corrosion resistance is improved by addition of small amount of antimony other than ordinary additives.

However it has been generally accepted that, as disclosed in Journal of Iron and Steel Institute (1939), antimony addition in small quantity would lower corrosion resistance even when with copper. The present inventors have found that addition of small amount of antimony to steel containing small amount of copper will improve the resistance against non-oxidizing acid due to the combined effects of copper and antimony. The present invention is based upon this new discovered fact.

The low alloy steels according to the present invention are particularly advantageous for use in the above environments.

Typical compositions of the corrosion resistant low alloy steels according to the present invention are as follows.

low cost hardening element. However steel containing carbon in excess of 0.80% is used as tool steel or steel for special purpose, and such steel is one which is not specially concerned with corrosion resistance. Therefore the upper limit of carbon content in the present invention is set 0.80%.

Silicon content is limited to not more than 1.00% because addition of more than 1.00% of silicon tends to lower the workability of the steel, although it, when present with copper, increases the corrosion resistance.

Manganese is effective for deoxidation of steel and increases workability and hardness, but less than 0.20% of manganese will not be enough for these effects. Therefore the lower limit of manganese content in the present invention is set 0.20% while the upper limit is set 1.50% because more than 1.50% of manganese has somewhat deteriorating effect on corrosion resistance.

Up to this day, the effects of sulphur content in steel on the corrosion resistance have not been well understood. The present inventors conducted experiments to investigate this. Specimens as shown in Table 1 were immersed in percent sulphuric acid at C. for four hours and their corrosion rate by weight loss were measured.

As clearly understood from the results of these tests, increased amount of sulphur will have deteriorating effects on corrosion resistance of steels in such strongly corrosive acid, and therefore the sulphur content in the present invention is limited to not more than 0.030%. However, the weight loss by corrosion in these tests can be lowered by increasing the additions of copper and antimony.

TABLE 1.EFFECTS OF SULPHUR CONTENTS ON CORROSION RESIST- ANCE (IN 40% SULPHURIC ACID AT 60 C. 4 HOURS) Weight; loss Sb (mg/cm O Si Mn Carbon Not more than 0.80%. Silicon Not more than 1.00%. Manganese 0.201.50%. Sulphur Not more than 0.030%. Phosphorus Not more than 0.030%. Copper 0.080.60%. Antimony 0.010.15%. Iron Balance.

Specimens:

Carbon Not more than 0.80%. Silicon Not more than 1.00%. Manganese 0.201.50%. Sulphur Not more than 0.030%. Phosphorus Not more than 0.030%. Copper 0.08-0.60%. Antimony 0.010.15%. Nickel and/ or chromium Not more than 1.00%

respectively.

Iron Balance.

Further, the corrosion resistant low alloy steel of the present invention may have modified compositions as obtained by adding to each of above two compositions not more than 0.40% of molybdenum, not more than 0.15% of vanadium, not more than 0.10% of titanium and not more than 0.10% of niobium in single or in combination.

Reasons for the above limitation of the additives in the present invention will be set forth as under.

As for carbon, carbon content may be determined according to the required properties of the steel, such as mechanical properties and weldability, since carbon is It is known that phosphorus when present with copper will increase atmospheric corrosion resistance and tensile strength, but excessive addition of phosphorus will embrittle the steel and tend to have very bad effect on resistance to sulphuric acid. Therefore phosphorus content in the present invention is limited up to 0.030%.

It is also conventionally known that copper is an element effective for increasing atmospheric corrosion resistance as well as corrosion resistance. However only a little increase in corrosion resistance can be expected when the addition of copper exceeds 0.80% while hot workability is lowered. Therefore the upper limit of copper is set 0.60%. The lower limit of copper is set 0.08% because no substantial improvement of corrosion resistance cannot be expected with less copper content.

Additions of nickel and chromium are effective for improving mechanical properties and atmospheric corrosion resistance and nickel, particularly, improves hot workability of the steel. However addition of nickel more than 1.00% will increase the cost of steel and lower somewhat corrosion resistance against sulphuric acid. And addition of chromium more than 1.00% Will lower somewhat resistance against sulphuric acid. Therefore the additions of nickel and chromium respectively are limited up to 1.00%.

C1 Sb Others Example 1 Table 2 shows the chemical composition of specimens, tensile strength, and the weight loss by corrosion after the h amount as to control deoxidation and grain size. Aluminium additions in such amount are negligible as for 10 their effect on the corrosion resistance, and therefore TENSILE STRENGTH Chemical composition Mn Cu sistance y, the eifect sion sion re dded singl TABLE 2 No. of specimens imony 1s a Tensile strength (kg/111.

S I 0 42 .2 360039462 h %11 .lwm .12231212 2 r 356 no 6 5 48683111303344 0049 h 321HUU1%W1%M2U%B212122332 111463 4 Corrosion rate by weight loss (mg/cm?) 17119023091 921 022089 111101211010011 111100 0000000000000000000000 QQQQQQQQQQQQQQQQQQQQQQ 3827874105851472130925 53484 35347458213567748 QQQQQQQQQQQQQQLQQQQQLQ 8634456540508753365083 0 7%27025203537374231333 3 568 68 070298091811983 100M10M111100101245677 QQQQQQQQQQOQQQQQQQQQQO No. of specimen:

the

1- 2- 3. 4. 5- s. 7. s. 9--. 10.. 11 12 13.- 14.. 1s 16 17 18 19 20- 21 improve th other maing steels. vanadium, titanium and niobium in combination to given in Table 2.

in comparison W1 :1 AL zm h m m eo I n r. W O m wm me ee i d m wa o t H 0 C m e 7 a H h m n OM 06 gamm S n S a ear mmm wra m n s mr5 m m .m 6 S e d S n W0 h aoiob IT I C V S a d f oenm ho f h 0 r. y .I e mot vrm t. hrna m n n maTmammO While a little improvement in corro can be obtained when ant of coexistance of antimony and copper on the corro resistance is remarkable and good corrosion resistance can be also obtained by the coexistance of antimony and arsenic in copper-contain Further molybdenum, may be added singly or mechanical properties and the weldability. However, these elements have generally deteriorating effect on the resistance to sulphuric acid and hydrochloric acid. Therefore it is necessary that additions of molybdenum, vanadium, titanium and niobium are not more than 0.40%, 0.15%, 0.10% and 0.10% respectively to obtain good mechanical properties and weldability as well as good acid resistance.

The present invention is further described in detail 0 through the following examples, referring to the attached drawings.

FIG. 1 shows the effects of various contents of antimony in the present corrosion resistant low alloy steels on the corrosion rate in 40% sulphuric acid at C. for 4 hours. The figures in FIG. 1 correspond to the specimen numbers FIG. 2 shows corrosion rates of the present corrosion resistant low alloy steels terials at various temperatures and concentrations of sulphuric acid induced from the vapour-liquid phase equilibrium in sulphuric acid-water system. The figures in FIG. 2 correspond to the specimen numbers shown in Table 3.

Specimens No. l-No. 25 in the table are steels of the present invention and their manufacturing processes such as steelmaking, ingot making and rolling operations were almost just same as the ordinary carbon steels and lowalloy steels and they were produced with great easiness and good yield. Specimen No. 26 is a steel to which has been added only antimony, but substantially no copper. As seen from its corrosion rate by weight loss remarkable improvement on the corrosion resistance is not obtained by additions of antimony alone. Specimens No. 27-No. 29 containing antimony more than 0.15% show better acid resistance, but not remarkably improved when compared with specimens containing about 0.15% of antimony, while their workability is deteriorated. Specimen No. 30 shows increased weight loss by corrosion due to the higher content of phosphorus. In this case the Weight loss can be reduced by increasing the contents of copper and antimony, but it is necessary to limit the phosphorus content less than 0.030% to obtain good acid resistance. Specimen No. 31 is a steel to which no antimony was added, and it is understood from the results of the experiment that sufficient acid resistance cannot be assured by additions of copper alone. Specimen No. 32 is a conventional corrosion resistant low-alloy steel (such as disclosed in Japanese Patent Gazette Sho 39/28011). Specimen No. 33 is a commercially available atmospheric corrosion resistant low-alloy steel, and specimen No. 34 is an ordinary carbon steel. Specimens No. 26-No. 34 have been shown for comparison with the present steel.

As clearly understood from the above results, the steels of the present invention have much better corrosion resistance than the ordinary carbon steels and conventional corrosion resistant low-alloy steels. The steels of the present invention show also good resistance against hydrochloric acid as well as they have large resistance against corrosion by non-oxidizing acids such as acetic acid. Steel having good corrosion resistance and atmospheric corrosion resistance can be obtained within the composition range as specified hereinbefore.

Example 2 Corrosion tests were conducted using specimens as shown in Table 3 which were immersed in sulphuric acid having specific temperatures and concentrations determined by the temperature-concentration correlation of sulphuric acid calculated from the vapour-liquid phase equilibrium of sulphuric acid-water system. The results are shown in FIG. 2.

the results shown in Table 4 were obtained from the tests in which specimens of the present steel and conventional steels were placed within an actual boiler to determine the resistance against corrosion by sulphuric acid in actual service which appears to be one of the problems to be met with in the applications of the present steels.

The tests were conducted as under.

TABLE 4.RESULTS OF CORROSION TESTS IN OUT-LET FLUE OF AIR FEE-HEATER OF HEAT POWER PLANT Rate of corrosion (X10- min/hour) No. 1 N o. 2 Specimen Specimen Average IVIiltl steel 1. 335 1. 207 1. 301 Corrosion resistant steel (A) 0. 901 0.932 0.917 Corrosion resistant steel (B) 0. 826 O. 898 0. 802 Present steel 0.737 0.707 0. 722

The results of the above tests in actual service have proved that the steel of the present invention has much greater resistance against the corrosion by sulphuric acid than the conventional steels.

As clearly understood from the foregoing descriptions the steels according to the present invention can be produced easily and at lower cost and have very excellent corrosion resistance, particularly acid resistance, and thus great industrial advantages and utilities.

What is claimed is:

1. Low-alloy steel resistant to non-oxidizing acids and consisting essentially of not more than 0.80% of carbon, 0.201.50% of manganese, not more than 1.00% of silicon, 0.01-0.15 of antimony, not more than 0.030% of sulphur, not more than 0.030% of phosphorus, 0.08 0.60% of copper, the balance being substantially iron.

2. Low-alloy steel as claimed in claim 1 further hav- TABLE 3.-GIE[EMICAL COMPOSITIONS OF SPECIMENS C Si Mn P 5 Cu Ni N 0. of specimen:

Specimen No. 4 and 16 in the table are steels of the present invention, No. 32 is a conventional corrosion resistant low-alloy steel, No. 33 is a conventional atmospheric corrosion resistant low-alloy steel, No. 35 is loW carbon steel and No. 36 is 18Cr-8Ni stainless steel.

Each of these specimens was subjected to the humersion test at a predetermined temperature for 4 hours. The results of these tests are shown in FIG. 2, which indicates that the steels of the present invention have better corrosion resistance under any of the testing conditions, and it is also indicated that the corrosion resistance of the present steels is particularly remarkable in the most corrosive range of 40% at 60 C.-% at 80 C.

Example 3 While the corrosion tests in Examples 1 and 2 were conducted in a laboratory under strict testing conditions,

ing not more than 1.00% of a metal taken from the group consisting of nickel, chromium and alloys consisting of nickel and chromium.

3. Low-alloy steel as claimed in claim 1 further having a metal taken from the group consisting of molybdenum in an amount not greater than 0.40%, vanadium in an amount not greater than 0.15%, titanium in an amount not greater than 0.10%, niobium in an amount not greater than 0.10%, and alloys of said metals in said group.

4. Low-alloy steel as claimed in claim 1 further having not more than 1.00% of a metal taken from the group consisting of nickel, chromium and alloys consisting of nickel and chromium, and further having a metal taken from the group consisting of molybdenum in an amount not greater than 0.40%, vanadium in an amount not greater than 0.15%, titanium in an amount not greater 9 than 0.10%, niobium in an amount not greater than FOREIGN PATENTS 0.10%, and alloys of said metals of said group. 424,781 2/1935 Great Britain.

References Cited HYLAND BIZOT, Primary Examiner UNITED STATES PATENTS 5 US Cl XR 2,121,057 6/1938 Sm1th 75125 128 2,369,656 2/ 1945 Briggs. 2,867,531 1/1959 Holzwarth. 

